Blender 2.8: any alternative for changing between modes other than the (uncomfortable) pie menu?
You can simply change the keymap in User Preferences. Or - the fast way - in the top left corner, where you have the dropdown for the modes: Click once to show all the modes, then right click on Weight Paint -> Assign Shortcut.

You can do that with almost everything in Blender 2.8.
Another Option is (in the same Context Menu) "Add to Quick Favorites". Then it will appear under the Menu that you get when pressing "Q". 

Hope that helps!